Analog cellular telephone systems have been long established in the United States. The advanced mobile phone service (AMPS) system which is still is widespread use today, was developed in the early 1970s. Frequency division multiple access (FDMA) is used in the AMPS system to provide shared access to the available spectrum. With FDMA, users are assigned to a channel from a limited set of channels in the available spectrum. In the AMPS system, 25 kHz channels are used whose carriers are separated by 30 kHz. The number of available channels is therefore limited by the available spectrum. When a user places a call, the user is assigned to one of the available channels. If no channel is available, the user's call is blocked. Unfortunately, the increasing demand for cellular services has resulted in a corresponding increase in the frequency of call blocking, particularly in congested urban areas.
Recent advances in digital communications allow for an increase in network capacity without requiring allocation of additional spectrum. Digital communication systems use time division multiple access (TDMA) or code division multiple access (CDMA) to allow multiple users to share the same carrier thereby increasing network capacity. In a TDMA system, for example, a single RF carrier is divided into time frames of a pre-defined length. Each time frame is further divided into plural time slots, each time slot representing a separate channel. A plurality of users can transmit and receive data in short bursts in respective time slots. Thus, a plurality of users can share a single carrier.
Another advantage of digital systems is the improvement in voice quality. In an analog system, physical influences or disturbances in radio transmission links are sometimes passed into the audio path of the receiver. The results are static, hums, hisses, crackling sounds, cross-talk and fade-outs in the received voice signal. In digital communication systems, the audio signal is transformed into digital data patterns called bits or bit patterns. Digital coding and error correction methods allow the analog signal to be reconstructed at the receiving end of the transmission. Digital signal processing techniques eliminate many of the effects of disturbances in the radio transmission link. Digital control of mobile phones also has advantages including longer battery life, short-message services, hierarchical cell structures, and private/residential service.
Because of the advantages of digital communication's techniques, there has been a move to replace existing analog cellular systems in favor of digital cellular systems. However, there is a huge investment of resources in the current analog infrastructure and it is not practical to completely abandon existing analog systems. Moreover, there are millions of existing analog-only mobile phones in use which require analog infrastructure to operate. The transition to digital systems will likely take many years or decades. Consequently, it can be expected that cellular services in the U.S. will consist of a mixture of analog and digital systems.
The IS-136 standard has been developed for dual mode cellular telephones which are operable in either an analog or a digital communications mode. In general, it is desirable to operate in a digital communication mode when digital services are available. When an attempt is made to acquire cellular services by a dual mode mobile telephone in a channel range where both analog and digital control channels are available, the mobile telephone will first search for a suitable analog control channel. After finding a suitable analog control channel, the mobile phone will wait for a digital channel pointer. The digital channel pointer is a pointer which directs the mobile phone to a digital control channel. Since digital services is preferred, the mobile phone will wait for a channel pointer before acquiring service on an analog control channel. If no pointer information is received after a predetermined period of time, the mobile telephone will acquire service on an analog control channel. Thus, in a dual mode phone, delay while waiting for a channel pointer to a digital control channel increases substantially the amount of time it takes to acquire service on a control channel and thus be able to send and receive messages.